1. Field of the Disclosure
The present invention is related to power converters. More specifically, the present invention is related to power converters that operate with high efficiency at light loads.
2. Background
Controllers for off-line power converters often must measure the input voltage to perform functions such as under-voltage detection and over-voltage protection. Off-line power converters typically receive an input voltage that is greater than 100 volts ac. Since the ac voltage varies periodically between a peak positive value and a peak negative value at the frequency of the power line, the ac line voltage is typically represented numerically as a root mean square (rms) value of a sine wave. The rms value of the ac voltage is the magnitude of the peak voltage divided by the square root of two. For example, in the United States the common household voltage is 120 volts ac with a peak value of 169.7 volts. In many other parts of the world, the common household voltage is 240 volts ac with a peak value of 339.4 volts. The ac rms voltage is equivalent to a dc voltage of the same numeric value when both are applied to the same resistive load such as an incandescent lamp. Transient disturbances and faults on the power line can momentarily raise the voltage to substantially higher values.
Off-line power converters typically rectify the ac input voltage to obtain an unregulated dc input voltage that is then converted to a lower regulated dc voltage. The maximum value of the unregulated dc input voltage is approximately the peak value of the ac input voltage. Semiconductor components in the power converter may need to withstand voltages that are substantially greater than the peak of the ac input voltage when the converter is operating. Therefore, it is necessary for controllers of power converters to measure the input voltage so that the components may be protected from damage due to excessively high voltage. A controller may halt operation of the converter to prevent damage when the input voltage goes higher than a threshold value.
Circuits that measure an input voltage typically do so by using a potential divider across the input to provide a known fraction of the input voltage that is low enough for the measurement circuit to handle. In order to reduce power consumption, the components of the divider are selected to take no more current from the input than necessary. To reduce power consumption further and to reduce the number of components, a current that represents the input voltage can be used instead of a potential divider. However, the current needs to be large enough to guarantee a reliable measurement in the presence of noise. The power taken from the source of input voltage is proportional to the product of the voltage and the current. Since the peak value of the ac input may be hundreds of volts, even the smallest current acceptable for reliable measurement can still result in a significant loss of power, especially when the power converter has a light load or no load. Power converters need a controller that can sense the input voltage reliably with low power consumption.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.